1. Field of the Invention
This invention relates to an automatic focus detection apparatus for performing focus detection using light projected from a light projection means.
2. Background of the Related Art
Heretofore, as a type of focus detection apparatus used for a camera and the like, there has been known an apparatus in which the deviation between two images formed by dividing the exit pupil of a photographing lens is observed by a focus detection optical system to determine the focus state. There has been disclosed, for example, a secondary imaging method in which two secondary optical systems are disposed at an expected focal plane of a photographing lens of a camera, and two images are generated which deviate from each other in accordance with a defocus amount of the photographing lens. The two images are guided to a surface of a photoelectric conversion sensor which detects the relative deviation in image position, as shown in Japanese Patent Public Disclosure (Kokai) Nos. 55-118019 (1980), 55-155331 (1980) and the like.
In automatic focus detection apparatuses having light projection means and using the method described above, focus detection using light projection is limited to when the brightness of an object to be photographed is low and hence contrast is small, such as in darkness and the like. This is because it is impossible to discriminate between external light and the light emitted by the light projection means. When there exists external light more than a certain degree, light projection by the light projection means cannot provide its proper effect.
In consideration of the disadvantage described above, the assignee of the present application has proposed, though not laid open yet, in Japanese patent application No. 63-183115 (filed Jul. 22, 1988) a method capable of solving the above-described disadvantage by using a system in which the emitted light and the external light are discrimiated from each other by time serially modulating a light-emitting source. This method will be briefly explained with reference to FIG. 7.
A light projection circuit 220 emits a pattern light projection (a pattern in which the illuminating light has a spatial distribution of brightness) in response to a transmitted control signal 1. An array SNS of photoelectric conversion devices performs signal storage while removing external light in the following way. Light incident upon photoelectric conversion devices 224 is subjected to photoelectric conversion, and the converted signal is stored in either a storage capacitance device 226 or a storage capacitance device 227 selected by a switch 225 controlled by the control signal 1. The light projection circuit 220 and the switch 225 are operated in synchronization with the control signal 1, and the signal is stored in the storage capacitance device 227 when light is projected and in the storage capacitance device 226 when light is not projected. A differential amplification circuit 228 outputs a signal corresponding to the difference between the storage capacitance devices 226 and 227. The output from the differential amplification circuit 228 is equal to the photoelectrically-converted output when light is projected minus the photoelectrically-converted output when light is not projected, that is, an output in which the external-light component is removed.
An image signal in which an external-light component is removed can be obtained via switches 229 controlled by a control signal 2.
As is apparent from the above description, the above-described application can solve the above-described disadvantage and the like, and hence provides a very effective apparatus. However, since the apparatus includes two storage capacitance devices for each picture element, and uses an amplification circuit for each sensor for outputting the difference between the two capacitance devices, and controls storage by switching between two storage capacitance devices in synchronization with the light projection by the light projection circuit, specific sensors and control circuit are needed.